fix ordering for logged out users

app/models/post.rb

@@ -72,11 +72,11 @@ class Post < ActiveRecord::Base
   }
 
   def self.next(post)
-    where("posts.created_at > ?", post.created_at).order('posts.created_at ASC').last
+    where("posts.created_at > ?", post.created_at).order('posts.created_at ASC').first
   end
 
   def self.previous(post)
-    where("posts.created_at < ?", post.created_at).first
+    where("posts.created_at < ?", post.created_at).order('posts.created_at DESC').first
   end
 
   def self.visible_from_author(author, current_user=nil)